The present invention relates to a surface-mounted choke coil, and more particularly to a surface-mounted choke coil with the smaller mounting area and the higher mounting density, in which the formation of a solder fillet can be easily confirmed at the mounting time.
The related-art surface-mounted choke coil is illustrated in FIGS. 3A to 3E. In the choke coil, a drum core 2 having a core rod is provided. The drum core 2 is almost rectangular in plan view with four corners cut away, and has a pair of external electrodes 3 fixed on its lower face. The solder fillet formation parts 3a also serving as drum core positioning members are extending on the lateral face of the drum core 2 from the external electrodes. The coil, not shown, is wound around the core rod of the drum core 2, with its both ends being connected via solder connection parts 4 to the one pair of external electrodes. A part of the coil is appropriately coated with a adhesive agent. The surface-mounted choke coil 1 is contained within a square area 5 as large as, e.g., 3×3 mm2, conforming to the prescribed size standard, including the solder fillet formation parts 3a. 
FIGS. 3C and 3D show the connection lands 7 to which the one pair of external electrodes are soldered when the surface-mounted choke coil 1 is directly mounted on a board 6. The connection lands 7 are formed so as to straddle the boundaries of the square area 5. As shown in FIG. 3E, when the external electrode 3 is soldered to the connection land 7, the molten solder rises up the solder fillet formation part 3a to form a solder fillet 8 between the solder fillet formation part 3a and the connection land 7.
This solder fillet 8 formed at the mounting time assures that the external electrode 3 is securely connected to the connection land 7. Therefore, there is a demand that the surface-mounted choke coil 1 allows the solder fillet 8 formed at the mounting time to be confirmed from the outside.
Japanese Patent Publication No. 2005-150470A discloses a surface-mounted choke coil comprising: a magnetic core having a core rod and a flange formed on one end of the core so as to have a T-shaped longitudinal cross section; a pair of external electrodes each of which extends from a lower face of the flange to a side face of the flange so as to have an L-shaped longitudinal cross section; a coil both ends of which are electrically connected to the external electrodes is wound around the core rod; a resin layer containing magnetic powder and covering the coil; and a silver paste layer and a solder plating layer which are provided on a lower face of each of the external electrodes and adapted to be used for surface-mounting.
The surface-mounted choke coil of as shown in FIGS. 3A to 3E is contained within the square area 5 conforming to the prescribed size standard, including the solder fillet formation parts 3a extending on the lateral face of the drum core 2. Therefore, the drum core 2 is deprived of the outer size by the thickness of the solder fillet formation parts 3a, with the volume of the drum core 2 being smaller. Since the performance of the choke coil depends on the volume of core, the surface-mounted choke coil is prevented from being improved in the performance. Further, to confirm the solder fillet 8 from the outside, the solder fillet formation part 3a is stood on the lateral face, the connection land 7 on the board 6 is formed so as to straddle the boundaries of the square area 5, and the solder fillets 8 are formed in a portion beyond the boundaries of the square area 5. Therefore, the mounting area is increased to impede the higher density mounting.
In the surface-mounted choke coil disclosed in Japanese Patent Publication No. 2005-150470A, since the external electrodes extend to the side face of the flange, the mounting area is increased as much as the thickness of the electrodes provided on the side face of the flange. Accordingly, the high density mounting is impeded due to the reason as described the above. In addition, the external electrodes are connected to connection lands formed on a board by the solder plating layer provided on the lower face of the external electrodes. Therefore, it is difficult to confirm the connection part with the solder plating layer.